The smells may drive some species to extinction, unless conservationists take unorthodox measures, such as adding "deodorant" to bird nests, according to biologist Jim Briskie of Canterbury University in Christchurch, New Zealand.

Many bird scents stem from a gland that produces waxes essential to keeping feathers healthy.

In Europe and the Americas, birds' bodies alter this preening wax during breeding season, changing the wax's composition to reduce smells and keep the birds' nests less detectable by predators that use their noses to find food.

In a recent experiment in New Zealand, Briskie compared waxes from six native species, such as robins and warblers, with waxes from invasive species, such as blackbirds and sparrows, which had evolved in Europe until the 1870s.

"The European birds in New Zealand changed their preen waxes to become less smelly in the breeding season," he said. "But native birds did not, and they remained more smelly overall," Briskie said.

For instance, native kiwis—flightless, chicken-size birds—smell like ammonia, and kakapo parrots, also flightless, smell like "musty violin cases," he said.

Other New Zealand species seem to have similarly distinctive scents, Briskie said, unlike most birds on other continents.

"We do know that it's easy for muzzled dogs to find kakapo and kiwi by their smell, so I suspect that predators like rats or feral cats might be able to easily find native birds also," Briskie said.

Alien Predators On the Scent

New Zealand's birds may be so pungent largely because they were able to get away with it for so many centuries, Briskie suspects.

When New Zealand split away from Australia some 80 million years ago, no predatory mammals came along for the ride, so native birds never had to evolve means of masking their scents to survive, he said.

But eventually humans changed the landscape. The native Maori people introduced the Polynesian rat, and Europeans later unleashed other rat species, domestic cats, and the stoat—a type of weasel—which have easily caught on to the birds' scents.

One solution could be to give deodorant to the odorous birds, Briskie said. (See bird pictures.)

"If we prove that this is a problem, we might be able to envision some kind of odor-eater or deodorant we could put into the nest to absorb some of those odors and protect them more effectively," Briskie said.

But there's a potential downside—the birds' stench may serve other purposes.